Fuel cells are a highly efficient power generating device, and have advantages in that the amount of fuel use is low due to high efficiency compared to existing internal combustion engines, and it is a pollution-free energy source that does not produce environmental pollutants such as SOx, NOx and VOC. In addition, there are additional advantages in that a locational area required for production facilities is small, and a construction period is short.
Accordingly, fuel cells have diverse applications covering a mobile power supply such as portable devices, a transport power supply such as vehicles, and dispersion power generation usable for households and electric power industries. Particularly, when an operation of a fuel cell vehicle, a next generation transportation device, is commercialized, the potential market size is expected to be extensive.
Fuel cells are largely divided into 5 types depending on an operating temperature and an electrolyte, which specifically includes an alkali fuel cell (AFC), a phosphoric acid fuel cell (PAFC), a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC), a polyelectrolyte fuel cell (PEMFC) and a direct methanol fuel cell (DMFC). Among these, a polymer electrolyte fuel cell and a direct methanol fuel cell having excellent mobility have received wide attention as a future power supply.
A polymer electrolyte fuel cell has a basic principle such that gas diffusing electrode layers are disposed on both surfaces of a polymer electrolyte membrane, and water is produced by a chemical reaction through the polymer electrolyte membrane by facing an anode toward a fuel electrode and a cathode toward an oxidation electrode, and the reaction energy produced therefrom is converted to electric energy.
A typical example of an ion-conducting polymer electrolyte membrane may include Nafion, a perfluorinated hydrogen ion exchange membrane developed by Dupont USA in early 1960s. Similar commercialized perfluorinated polymer electrolyte membranes other than Nafion include Aciplex-S membrane manufactured by Asahi Kasei Chemicals Corporation, Dow membrane manufactured by Dow Chemical Company, Flemion membrane manufactured by Asahi Glass Co., Ltd., and the like.
Existing commercialized perfluorinated polymer electrolyte membrane has chemical resistance, oxidation resistance, and excellent ion conductivity, but has a problem of high costs and causing environmental problems due to the toxicity of intermediates produced during manufacture. Accordingly, polymer electrolyte membranes in which a carboxyl group, a sulfonic acid group or the like is introduced to an aromatic ring polymer have been studied in order to compensate for the weaknesses of such perfluorinated polymer electrolyte membranes. Examples thereof include sulfonated polyarylether sulfone [Journal of Membrane Science, 1993, 83, 211], sulfonated polyetherether ketone [Japanese Patent Application Laid-Open Publication No. H06-93114, U.S. Pat. No. 5,438,082], sulfonated polyimide [U.S. Pat. No. 6,245,881] and the like.
A polymer electrolyte membrane accompanies changes in membrane thicknesses and volumes of 15% to 30% depending on a temperature and a degree of hydration, and accordingly, the electrolyte membrane is repeatedly expanded and contracted depending on the operation condition of a fuel cell, and microholes or cracks develop due to such volume changes. In addition, as a side reaction, hydrogen peroxide (H2O2) or peroxide radicals are generated from a reduction reaction of oxygen in a cathode, which may cause the degradation of the electrolyte membrane. A polymer electrolyte membrane for a fuel cell has been developed in the direction of improving mechanical and chemical durability keeping such a phenomenon that may occur during the fuel cell operation in mind.
Studies that have been carried out for improving mechanical durability include a reinforcing composite electrolyte membrane prepared by introducing a Nafion solution (5% by weight concentration) to an e-PTFE (U.S. Pat. No. 5,547,551), and a polymer blend composite membrane introducing a polymer having excellent dimensional stability to a sulfonated hydrocarbon-based polymer material (Korean Patent No. 10-0746339), and the like. In addition, W.L. Gore & Associates introduces a reinforcing composite electrolyte membrane product commercialized as a trade name of Gore Select.
In a reinforcing composite electrolyte membrane, a porous support is used in order to provide mechanical properties and dimensional stability. A porous support needs to maintain mechanical durability while not declining performance, therefore, a support made of suitable materials provided with high porosity and excellent mechanical properties needs to be selected. In addition, ion conductivity of a membrane may greatly vary depending on the method of immersing an ion conductor into a support and the type of the ion conductor, therefore, development of an effective method of immersing an ion conductor, and an ion conductor suitable for a reinforcing composite electrolyte membrane has been required.